[gdal-dev] Under what (if any) conditions would GDALWriteBlock modify data buffer?

[Andrew C Aitchison]

On Mon, 18 Dec 2023, Even Rouault wrote:

>
> Le 18/12/2023 à 21:15, Andrew C Aitchison a écrit :
>> On Mon, 18 Dec 2023, Even Rouault via gdal-dev wrote:
>> 
>>> 
>>> Le 18/12/2023 à 16:18, Andrew C Aitchison via gdal-dev a écrit :
>>>> On Mon, 18 Dec 2023, Even Rouault via gdal-dev wrote:
>>>> 
>>>>> Hi,
>>>>> 
>>>>> interesting question. No easy answer as it is highly driver dependent. I 
>>>>> believe that all drivers make sure that the content of the buffer before 
>>>>> and after the call is the same, but some drivers might temporarily 
>>>>> modify it, to do byte swapping. For example the HFA driver does that 
>>>>> when run on big-endian hosts for non-Byte data type. I wouldn't exclude 
>>>>> that for formats with MSB-byte ordering, a similar situation would 
>>>>> happen for little endian hosts. So it is definitely not safe to use 
>>>>> WriteBlock() with a buffer that would come from a read-only section of 
>>>>> the calling program. Doc updated to reflect that in 
>>>>> https://github.com/OSGeo/gdal/commit/ea321723dfc69ef3a422b1e3fe4dc9ee0832861d
>>>> 
>>>> Did you mean to say
>>>>   Note that even with eRWFlag==GF_Write, the content of the buffer
>>>>   might be temporarily modified during the execution of this method
>>>>   (and eventually restored back to its original content), so it is not
>>>>   safe to use a buffer stored in a read-only section of the calling
>>>>   program.
>>> 
>>> Yes I meant that. I've less evidence in the RasterIO(GF_Write, ...) case 
>>> than in the WriteBlock() case, but without checking all drivers, it is 
>>> more prudent to assume that the buffer might be touched during 
>>> RasterIO(GF_Write) by some drivers.
>> 
>> In that case I don't understand
>>    Note that even with eRWFlag==GF_Write...
>> It suggests to me that the buffer may be modified for eRWFlag==GF_Write
>> *as well as* for eRWFlag==GF_Read (which implies that it is *more* likely
>> to happen for GF_Read) ?
>
> Ah, perhaps non-idiomatic wording from mine. I struggle to make that clearer: 
> if you've better wording to suggest, that's welcome

Ah. I had misunderstood.
I thought we were reading to or writing from *the buffer* not the raster.
The text is fine.

-- 
Andrew C. Aitchison                      Kendal, UK
                    andrew at aitchison.me.uk